1. Field of the Invention
The present invention relates to a cache storage system and method.
2. Background
For improved data storage and management, a disk subsystem may present multiple virtual storage devices or volumes to a user, while employing multiple physical disk storage devices or volumes for actual storage of the user's data. In that regard, for a given virtual device configured on a disk subsystem, a single virtual track is identified by (i.e., named) a Virtual Track Address (VTA) and has a physical location where the data for the track is stored on the back-end at a physical disk storage device.
The efficiency of such subsystems has been improved using a unique copying mechanism, which may be referred to as “snapshot” copying. Snapshot copying is described in detail in U.S. Pat. No. 6,038,639 entitled “Data File Storage Management System For Snapshot Copy Operations,” which is assigned to the assignee of the present application and which is hereby incorporated by reference. Implemented in a disk subsystem, rather than creating an additional copy of the data itself, the snapshot mechanism provides for copying only the pointers associated with the data. Thus, there are multiple names in the virtual world for the same physical data object.
For example, suppose Virtual Track Address (VTA) “X” maps to a data object stored on back-end devices at location “A.” Further suppose that Virtual Track Address “Y” maps to a data object stored on back-end devices at location “B.” A snapshot operation performed from VTA “X” to VTA “Y” creates the ability to access the data object stored at location A by either name “X” or name “Y.” Such dynamic mapping of where data objects are found may be implemented through the use of a Log Structured File System, or other known dynamic mapping mechanisms. As a result, there are two tracks in the virtual world but only a single copy of the data in the physical world. It is the virtualization of storage that makes the snapshot copying feature possible in disk subsystems. The snapshot feature allows the same physical track to be accessed from multiple virtual track locations. One of the benefits of this form of replication is that the multiple copies of a virtual track do not require any additional physical space for the copies. In other words, one track is the same as a million tracks when it comes to space consumption of physical storage.
This benefit in space consumption, however, only applies to the space on the physical disk drives that make up the disk subsystem's physical storage. A limitation exists with the snapshot feature when a million “snapshot” tracks (i.e., one million copies of the same track) are read into the cache memory of the disk subsystem.
In that regard, the management of track images in cache memory systems is significantly different from the management of disk memory subsystems. More particularly, cache memory subsystems are divided into units, which may be referred to as segments, that are allocated to store the contents of a track when staged into the cache. Since there is no performance penalty for accessing different locations in cache memory as there is for storing tracks at different locations on a disk, a track will occupy whatever segments are available. Typically, a discontiguous set of cache segments holds the track contents. There is a structure, such as a directory, that identifies or lists the set of cache segments used for storing a particular track.
However, in the cache memory, each track occupies its own space and the amount of cache needed to hold one million copies of the same track is one million times the size of the original track. As a result, there exists a need for a cache storage system and method that provides a space consumption benefit in the cache memory of a storage system, such as a disk subsystem, similar to the benefit provided by snapshot copying in the physical disk storage devices of a disk subsystem. That is, there exists a need for cache storage system and method that allows cache segments holding track contents to be shared when the tracks are copies of each other.